1. Field of the Invention
This invention relates to line protectors of the type used to protect telephone lines from spurious voltages and spurious currents.
2. Description of the Prior Art
Devices for protecting telecommunications equipment against spurious voltages and currents are generally called station protectors. There are many types of station protectors currently in use.
One such type is the R123 station protector sold by assignees Reliable Electric/Utility Products operating unit. That protector is shown in FIG. 1. Protector 1 includes a molded phenolic base 2 which has brass terminal posts 3, 4, 5. Terminal post 3 is for connection to ground, while terminal posts 4 and 5 are for connection to the two lines, also known as tip and ring, of the telephone wires entering the subscriber's premises. Protector 1 also includes screw in arresters 6, 7 each associated with a respective one of lines 4, 5. Arresters 6, 7 may be either of the carbon or gas tube type. While not shown in FIG. 1 is it well known that protector 1 also includes a ground plate in base 2.
One example of an arrester of the gas tube type is shown in U.S. Pat. No. 4,241,374 entitled "Surge Voltage Arrester With Ventsafe Feature"(hereinafter the '374 patent), which is assigned to the same assignee as is the present invention. FIG. 1 of the '374 patent is reproduced herein as FIG. 2 with essentially the same reference numerals as used in the '374 patent. The arrester shown therein includes a primary arrester which utilizes a gas tube 22 and a secondary arrester which uses an air gap 42. The arrester is contained in a sheet metal housing 12. The sidewall 14 of the housing 12 is formed with a screw thread 18 which starts at shoulder 16 of housing 12 and ends before the open end of housing 12. Screw thread 18 allows the arrester to be threaded into the associated well 8, 9 of protector 1. Well 8, 9 a portion of which is shown in FIG. 2 has an internal thread for receiving thread 18. The arrester 6, 7 can be unscrewed from its associated well 8, 9 if the arrester has shorted to ground as for example as a result of the melting of solder pellet 56. As is well known in the art, the air gap 42 of the arrester of the '374 patent provides a desired secondary surge arrester in the event of the failure of the gas tube.
Another type of station protector is that shown in U.S. Pat. No. 4,710,846 (hereinafter the '846 patent). The protector described therein is a modular unit in which two arresters are inserted. Line terminals project upwardly from the housing. A ground clip has tines which penetrate the base of the housing to hold the arresters in place. As the unit is sealed by potting compound the entire unit must be replaced if the solder pellet in either arrester melts to thereby ground the associated line.
Each arrester in the modular unit of the '846 patent is in a plastic cylindrical enclosure such that one electrode is in contact with the head of the associated line terminal and the other electrode is brought into contact with the ground clip by the associated spring and the base cup. As is well known the operation of a surge arrester results in the generation of heat. That heat should be dissipated in order to not have any potential adverse affect on the operating life and breakdown characteristics of the surge arrester. The plastic cylindrical enclosure may not allow for adequate heat dissipation. In addition, the terminal which projects outwardly from each arrester well is said to be designed so as to form a substantially air tight seal with the well.
The modular unit described in the '846 patent has a smaller base than protector 1 of FIG. 1. That smaller base may make the modular unit more suitable than protector 1 for use in many applications where physical size of the protector may be important. One such application is in the network interface devices that telephone operating companies are installing at subscriber premises.
It is desireable that a station protector be smaller than either protector 1 or the modular unit of the '846 patent. It is further desireable that the arresters used in such a protector be capable of being interconnected to each other and to ground in a variety of configurations such that maximum usage can be made of available surface area, i.e., the station protector takes up the smallest area possible. It is also desireable that each surge arrester have better heat dissipation than the arresters of the '846 patent. The protector of the present invention meets those requirements as it is comprised of two or more modular line protectors which can be arranged in a variety of configurations. In each modular line protector the surge arrester is in a conductive enclosure which is exposed to air i.e., the enclosure is not further enclosed in a nonconductive material. Thus, the modular line protector of the present invention dissipates heat faster than the arresters of the modular unit described in the '846 patent.